The effect of thinning to three, four, five, or six lateral shoots on the growth parameters of five differently colored poinsettia (Euphorbia pulcherrima) cultivars (‘Christmas Star’, ‘Christmas Feelings’, ‘Cortez Candy’, ‘Marble Star’, and ‘Primero Roso’) was studied by consecutive measurements of growth from September to December. In a separate experiment, the effect of pot size (13-, 15-, or 18-cm diameter) on growth of ‘Christmas Star’ was studied. Cultivars responded similarly to thinning, but during the growing season, apparent differences in vegetative growth parameters between cultivars were observed, with ‘Primero Roso’ evaluated as the most vigorous. At the single-shoot level, the increased number of lateral shoots decreased the number of nodes at the beginning and the stem diameter at the end of the experiment. Leaf area, dry weight of stems, and total plant weight increased as the number of laterals increased at the end of experiment. The most distinctive canopy attribute in this study was the bract color because, according to customer evaluation, significant differences in visual appearance among tested cultivars were observed only for that trait. Pot size differentially affected the vegetative growth parameters of poinsettia ‘Christmas Star’. Increasing pot size from a 13- to 18-cm diameter resulted in an increase of plant canopy diameter, stem diameter, and the number of nodes on lateral shoots. There were no other substantial benefits on the measured growth parameters that could justify the use of pots larger than a 13-cm diameter for a poinsettia crop established at the beginning of September.